As one type of an antenna used in a high-frequency band equal to or more than a microwave, there is a patch antenna.
The patch antenna is referred to also as a microstrip antenna and is a generic term for antennas formed by using a conductor subjected to printed wiring on a dielectric substrate. The patch antenna features low production cost.
An antenna in which high directivity is produced by arranging a plurality of antenna elements on a planar surface is specifically referred to as a patch array antenna among various types of patch antennas. In a patch array antenna, a signal having a phase or an amplitude different for each antenna element thereof is provided, and thereby directivity can be changed. Therefore, a patch array antenna is often used for military applications in old times and for an antenna for a car radar and the like in recent years.
As a method for controlling directivity of a patch array antenna, a method in which each antenna element of a patch array antenna is connected with a phase shifter and a variable attenuator and these are controlled is most common.
PTL 1 illustrates, in FIG. 1 thereof, for example, a phased array antenna used as an antenna to be tested (a transmission antenna). The illustrated phased array antenna includes first to Mth (M is an integer equal to or more than 2) antenna elements, first to Mth variable attenuators, and first to Mth phase shifters, connected to the elements, respectively. The phased array antenna further includes a variable attenuator control circuit and a phase shifter control circuit. The variable attenuator control circuit controls each variable attenuator. The phase shifter control circuit controls each phase shifter.
Further, PTL 2 illustrates, in FIG. 4 thereof, a receiver used for a millimeter wave band wireless communication system. The illustrated receiver includes a plurality of unit reception circuits of an intermediate frequency (IF) band, including a plurality of antenna elements, respectively, and a plurality of variable attenuators and a plurality of variable phase shifters connected to these circuits, respectively. A control circuit, not illustrated, controls each variable phase shifter by a phase control signal and controls each variable attenuator by an amplitude control signal.
Further, PTL 3 illustrates, in FIG. 1 thereof, a small-size array antenna in which a direction of a beam of a radio wave is variable. The illustrated array antenna includes a plurality of antenna elements arranged on a substrate, a plurality of variable phase shifters connected to these elements, respectively, and a controller connected to each variable phase shifter. The controller controls each variable phase shifter.
In the methods of PTLs 1 to 3 described above, it is necessary to add an active element such as a phase shifter to a radio frequency (RF) circuit for each antenna element. Therefore, in the methods described above, when a directional gain is intended to be improved by increasing the number of antenna elements, active elements such as phase shifters proportional to the number of antenna elements are needed. Therefore, in the methods described above, there is a disadvantage that a circuit size of an RF circuit increases.
As another method for controlling directivity of a patch array antenna, a method for electronically controlling a reactance of a variable reactance element mounted on a dielectric substrate where a patch array antenna is formed has been proposed.
PTL 4 illustrates, in FIG. 1 thereof, for example, an array antenna device capable of electrically switching directivity. The illustrated array antenna device includes first to third slots formed parallel to one another on a conductor formed on a dielectric substrate, a power feeding unit mounted on each of the first to third slots, and first and second varactor diodes. The array antenna device changes capacitances of the first and second varactor diodes, and thereby controls directivity.
Further, PTL 5 illustrates, in FIG. 1 thereof, a planar array antenna including a single layer configuration. The illustrated array antenna device includes an active element formed on a dielectric substrate and first and second patch elements formed adjacently to the active element. The active element is provided with an RF signal source. First and second parasitic patch elements are connected with first and second variable reactance RF units, respectively. In the planar array antenna, reactances of the first and second variable reactance RF units are electronically changed, and thereby directivity is changed.
Further, PTL 6 illustrates, in FIG. 23A thereof, a variable directivity antenna device in which two antenna elements are formed on a dielectric substrate and a parasitic element connected with a P-intrinsic-N (PIN) diode is formed adjacently thereto. In the antenna device, whether or not the PIN diode is grounded is controlled, and thereby directivity is controlled.
In the methods of PTLs 4 to 6 described above, a circuit that controls directivity is formed on a dielectric substrate where an antenna is formed, and therefore a circuit size of an RF circuit itself does not increase. However, in the methods of PTLs 4 to 6, it is necessary to mount variable reactance elements proportional to the number of antenna elements on a dielectric substrate where an antenna is formed. Therefore, in the methods of PTLs 4 to 6, there is a disadvantage that, when a high directivity gain is intended to be obtained by increasing the number of antenna elements, a cost of an antenna increases.
As another method for controlling directivity of a patch array antenna, a method for controlling directivity by changing a position of a dielectric component has been proposed. In the method, a dielectric component is disposed on a microstrip line formed on a dielectric substrate and a position of the dielectric component is physically moved, whereby a phase of a signal passing through the microstrip line is changed. Thereby, directivity of a patch array antenna is changed.
PTL 7 illustrates, in FIG. 7 thereof, for example, an array antenna using a phase shift device capable of easily changing directivity. The illustrated array antenna includes two patch antennas, a power feeding line connected with these antennas, and a dielectric phase shifter disposed in a vicinity of the dielectric line.
The dielectric phase shifter includes a dielectric and a movement mechanism that moves the dielectric. In the array antenna, the dielectric is moved and thereby a phase of the patch antenna is changed, whereby directivity is changed.
In the method described in PTL 7, there is a disadvantage that it is necessary to physically move a dielectric component and therefore durability of a dielectric phase shifter is low.
As another method for controlling directivity of a patch array antenna, a method using a variable dielectric substrate has been proposed.
PTL 8 proposes, for example, an array antenna based on a phase shifter adjustable by a voltage, in which a low-loss dielectric material is adjusted by an applied voltage. In the proposed array antenna, a dielectric substrate is formed by using a material in which permittivity is electrically variable, and a phase of a signal passing through a microstrip line formed on the dielectric substrate is changed by controlling an applied voltage to the dielectric substrate. Thereby, directivity is changed. PTL 8 exemplifies barium strontium titanate, a liquid crystal, and the like as a material in which permittivity is electrically variable.
In the method of PTL 8, there is a disadvantage that it is necessary to use a special material for a dielectric substrate.
As another method for controlling directivity of a patch array antenna, a method using a variable power distributor has been proposed.
PTL 9 illustrates, in FIGS. 1 and 3 thereof, for example, a directivity variable antenna in which a power applied to each circular array of circular arrays formed double is changed by using a variable power distributor and thereby directivity is changed.
Further, PTL 10 has proposed an array antenna capable of controlling a polarization plane while not being a technique for controlling directivity. In the proposed array antenna, similarly to PTL 9, by using a variable power distributor, a distribution ratio of signal powers input from two power feeding points connected with a plurality of antenna elements is changed. Thereby, a polarization plane is controlled.